Warpage control in molded pulp articles



Nov. 15, 1966 J. E. FOOTE 3,284,917

WARPAGE CONTROL IN MOLDED PULP ARTICLES Filed June 12, 1965 FIG, 1 HOT AIR FLOW /0 PULP MOLD i FREE DRY TO 45"50/o WATER r FOR M .D RY TO 2-5% WATER INVENTOR JAMES E. F0075 KARL (U F ATTORNEY Patented Nov. 15, 1966 3,284,917 WARPAGE CONTROL IN MOLDED PULP ARTICLES James E. Foote, Westport, Conn., assignor to Diamond International Corporation, a corporation of Delaware Filed June 12, 1963, Ser. No. 287,378 12 Claims. (CI. 3421) The present invention relates to warpage control in molded pulp articles and more particularly to a system for drying molded pulp articles which prevents the Warpage therein.

Up until the present time it has been most conventional to dry molded pulp articles completely in shaped heating dies which restrain the articles. This restraining action causes the stresses that develop from the shrinkage of the fibers on drying to be relieved by fiber to fiber slippage which in turn results in a stress-free and thus warpfree article. This, however, is slow and expensive. It is also common to completely free-dry the pulp articles. This, however, results in some warpage and can be used only where a degree of warpage in the article can be tolerated.

The desirability of some free-air-drying of a freshly molded wet pulp article prior to its being dried by accurately shaped heated dies, which accomplish final drying with no distortion in the article, has been previously recognized. Even these methods have not been entirely successful, however, because prior to the present invention, the critical relationships between free and formdrying have not been recognized.

In accordance with the present invention it has been discovered that pulp molded articles can be dried without warping by free-drying to 45 to 50% water content and then form-drying to 2 to 5% Water. It has been discovered that a critical moisture content relationship exists between the percent of free-air-drying and the percent of form-air-drying. Thus, while the prior art recognized that free-air-drying followed by form-air-drying is desirable, it did not recognize that the free-air-drying cannot be carried out to less than 45% water (based on the bonedry article) and that the form-air-drying must be carried out to lower than 5% water (bone-dry basis). If either of these conditions are not met the product produced will be warped. To accomplish the above form-drying the present invention also includes novel stacking and drying procedures and drying forms which are improvements over the drying forms of the prior art.

It is therefore an object of the present invention to provide a novel process of drying pulp molded articles which is both effective to prevent Warpage yet it is relatively inexpensive and fast.

It is therefore another object of the present invention to provide novel and improved drying forms which are simple and inexpensive, yet are capable of form-drying pulp molded articles to prevent their warping.

It is another object of the present invention to provide a new and improved method of assembling pulp molded articles and drying forms.

It is another object of the present invention to provide a method for the positive control of warpage utilizing a combined system of free-drying and form-drying.

It is another object of the present invention to provide neW and improved drying forms.

It is another object of the present invention to provide a new and improved process of drying pulp molded articles.

It is another object of the present invention to provide a new and improved form and molded pulp article assembly for use in drying the molded pulp articles.

These and other objects and the nature and advantages of the instant invention will be apparent from the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a sectional view taken through an article and one embodiment of a drying form of the present invention;

FIG. 2 is a sectional view similar to FIG. 1 showing another form of the present invention;

FIG. 3 is a fiow diagram of the drying process of the present invention.

A vacuum drying form 19 shown in FIG. 1 comprises a backing plate 12 through which a suitable pipe 14 is passed which is connected to a source of vacuum (not shown). Facing the backing plate 12 is a main body portion 16 shaped to conform to a pulp molded article 18 desired to be dried. Body portion 16 of the vacuum drying form 10 contains holes 20 passing therethrough from the outer surface 17 to the inner surface 19. A screen member 22 is provided adjacent the outer surface 17 of the body portion 16 and a peripheral member 23 serves to clamp the screen 22 in place.

The vacuum drying form 10 of FIG. 1 operates in the following manner:

The pulp molded article 18 is placed over the screen 22 as shown in the FIG. 1 and a vacuum is applied through pipe 14. The form is passed through an oven and while therein hot air is pulled through the article 18, laterally along the screen 22, through the holes 20 and then out pipe 14. As the hot air passes through the article 18 it evaporates water therefrom and the hot moisture laden air is withdrawn through pipe 14. The function of the screen 22 is to permit uniform application of vacuum along the entire article 18 via each separately spaced hole 20. The force of the vacuum pulling against articles 18 maintains the article in place and prevents any warping in the article, i.e., the vacuum pressure force applied to the article 18 is greater than the strain forces created by the drying of the fibers in the article 18.

FIG. 2 shows a drying assembly utilizing drying forms 24 containing holes 20' passing therethrough. The forms 24 are preferably stamped from sheet metal such as aluminum and thus have a high heat conductivity coefiicien-t. Each individual drying form has a concave surface 26 and a convex surface 28. The convex surface 28 is shaped to conform to the inner concave surface of the pulp molded article 18 and thus the shapes of the pulp molded article 18 and individual drying form 24 are complementary. Each of the drying forms 24' has an outwardly flaring wall 30 which extends a distance greater than the complementary outwardly flaring wall of the pulp molded articles 18. The inner peripheral length of each drying form 24 at the extreme inner end of its flaring wall 30, as shown at points 32, is the same length as the outer peripheral length of the article 18 at the extreme outer length of its outwardly flaring wall, as shown at 34. The result of this configuration is that as the individual drying forms 24 and the pulp molded articles 18 are stacked as shown in FIG. 2, a pinching force is exerted between two individual drying form 24 on an article 18 clamped therebetween at the peripheral line 34. This in turn exerts a transverse tension or stretching force along the article 18 as shown by the arrows F and this force is greater than the strain forces created by the drying of the fibers; this pinching force thus prevents warpage as the articles are being dried.

The procedural assembling of the FIG. 2 embodiment may be effected by either placing one article 18 on one form 24 and then placing a second form on top of the article thus procedurally alternating one layer of form followed by a layer of article. On the other hand, each article 18 may be independently placed on each form 24 and then the pairs assembled. If desired, combinations of these methods may be used. Whichever way the stack is assembled the pulp molded articles 18 are dried merely by forcing hot dry air through the holes 20 in the individual forms 24. As the hot air rushes by and/ or through the articles 18, it causes evaporation of moisture therefrom.

In accordance with the method of the present invention it is desirable upon removing the pulp molded articles from the pulp mold, to free-dry the pulp articles to the moisture content as low as possible. In accordance with the present invention it has been found that the freeair-drying may reduce the moisture content to as low as 45% based on a bone dry article. If dried below 45 then warpage will have begun to take place and subsequent form-drying will not cure the warped appearance of the article. If the air drying is stopped short of 50%, on the other hand, the economic advantages inherent in the free drying will not be fully attained. Thus it is desirable to free-air-dry to between 50 and 45 moisture content. At this point the free-dried pulp molded articles should be placed in the drying forms shown in FIGS. 1 and 2 and form dryed to a moisture content of between 2 and If the articles are not dried on the forms to a moisture content of less than 5%, then the articles will not be stable and will have a tendency to Warp. The 2% moisture content, on the other hand, is a practical limit to which drying may be carried out, it being commercially impractical to dry the article to below 2% moisture. Free-drying to the 50% moisture level also provides the advantage that the pulp molded articles at that moisture content possess sufiicient strength to withstand reasonable mechanical handling without undue distortion.

In the following examples the moisture percents are calculated on the basis of a bone-dry article.

Example 1 A series of rectangular pulp molded trays are free-airdryed from a moisture content of 230% to a moisture content of 70%. The free drying time is 300' seconds. These free-dried pulp molded articles are then placed in an assembly such as shown in FIG. 2 and hot dry air is forced through the assembly. After 100 seconds the moisture content is reduced to 4.5%. The resultant articles are essentially warp free.

As a control, similar pulp molded articles are freedried in a similar manner and then form-dried in the apparatus shown in FIG. 2 for only 50 seconds. The resultant moisture content is 9% and upon setting the articles become warped.

Example 2 The procedure of Example 1 is carried out except that the form of FIG. 1 is used .and vacuum is applied therethrough. To obtain a moisture content of 4.5% in the final article, only 40 seconds of form drying time are required. The resultant articles are essentially warp-free.

Example 3 The procedure and form used in Example 1 is repeated except that the free air drying is extended to reduce the moisture content to 50%; this requires 435 seconds. The resultant articles, form-dried in accordance with Example 1, are dry and free of warpage.

Example 4 The procedure and form of Example 1 are again utilized-except that the free air drying is carried out to a moisture content of 46%; this takes 445 seconds. The articles produced after form-drying in accordance with Example 1, are warp free.

Example 5 Once again the practice of Example 1 is carried out except that this time the free air drying is extended to 40% moisture level. This takes 460 seconds. The resultant articles are wanped and the form-drying in accordance with Example 1 does not alter the warped condition.

Example 6 A series of rectangular pulp molded trays are free-air dried from a moisture content of 230% to a moisture content of 48%; this requires 440 seconds. The free-dried pulp articles are then placed in the assembly of FIG. 2 and hot dry air is forced by the assembly. Half of the articles are form-dried for seconds to a moisture content of 6%, while the remaining articles are form-dried for seconds to a moisture content of 4%. The articles are removed from the form and after reaching equilibrium, the first batch becomes warped and the second remains substantially warp-free.

Example 7 A series of rectangular pulp molded trays are free-airdried from a moisture content of 230% to a moisture content of 42%; this takes 455 seconds. The free-dried pulp articles are then placed in vacuum drying forms as shown in FIG. 1; hot, dry air is drawn through the articles for 30 seconds reducing the moisture content to 4.1%. The resultant articles are wanped.

It will be obvious to those skilled in the art that various changes may be made without departing from the scope of the invention and therefore the invention is not limited to what is shown in the drawings and described in the specification but only as indicated in the appended claims.

I claim:

1. A method of assembling free dried pulp molded articles and drying forms for the form drying of the pulp molded articles to prevent warping comprising placing the concave surface of a pulp molded article adjacent the complementary convex surface of a thin perforated drying form, placing a second drying form over the article to contact and apply a pinching force to the article along only a small area of the convex surface of the article adjacent its extreme end to exert a stretching force along the surface of the article, and assembling in a stack a plurality of forms and articles in alternating relationship.

2. A method in accordance with claim 1 wherein the pulp molded article has previously been free-dried to a level wherein the moisture content is no less than 45%.

3. A method in accordance with claim 1 where the total number of forms in a stack is one greater than the number of articles.

4. A method in accordance with claim 1 wherein the articles are placed on the forms prior to stacking and each pair of form and articles are then stacked.

5. A method in accordance with claim 1 further comprising the step of forcing hot air through the perforated forms to dry said articles to a moisture content of less than 5% water.

6. A method of drying pulp molded articles comprising removing the articles from the forming molds, free drying the articles without restraint to a moisture content of no less than 45% water, placing the articles in restraining drying forms to inhibit warping during further drying, contacting the restrained articles with hot dry air until the articles have been dried to a moisture content of less than 5% Water, and removing said articles from said drying forms and permitting the articles to reach equilibrium without warpage.

7. A method in accordance with claim 6 wherein the free drying is carried out to a moisture content of between 45% and 50% water.

8. A method in accordance with claim 6 wherein the form drying is carried out to a moisture content of between 2% and 5% water.

9. A method of drying pulp molded articles comprising free drying the articles without restraint to a moisture content of no less than 45 water, placing the concave surface of an article adjacent a complementary convex surface of a thin perforated drying form, placing a second drying form over the article to contact and apply a pinching force to the article along only a small area of the convex surface of the article adjacent its extreme end to exert a stretching force along the surface of the article, assembling a plurality of forms and articles in alternating relationship in a stack, and contacting the restrained articles 'with hot dry air until the articles have been dried to a moisture content of less than 5% water.

10. A drying system comprising a plurality of perforated drying forms; an article having a bottom wall and side walls located between each pair of forms; and means for forcing a flow of hot air through said forms to effect drying of said articles; each of said forms comprising a thin perforated sheet convexly shaped to complement and contact the inner concave surface of said article to be dried, said form further comprising a slightly outwardly flared wall extending a length greater than the length of the outwardly extending side wall of said article desired to be dried, said slightly outwardly flared wall having an inner peripheral length at its extreme end equal to the outer peripheral length of the article to be dried at the articles extreme end, said forms and articles being so stacked as to provide an air space between the concave surface of each form and the convex surface of the next adjacent article, the height of said air space being of the magnitude of the height of said article side wall.

11. A drying assembly comprising a stack of alternate pulp molded articles and perforated drying forms, the

total number of said drying forms being one greater than the number of said articles, said articles being in the form of trays having a flat bottom and an upwardly and outwardly extending wall, said forms each comprising a thin perforated sheet being complementary in shape to said articles and comprising a wall which extends in length a greater distance from the form bottom than said article wall extends from said article bottom, said form wall having an inner peripheral length at its extreme end equal to the outer peripheral length of said article wall at said article walls extreme end, said assembly being stacked so that a form on one side of an article is in contacting complementary relationship to the article while a form on the other side of the article contacts and applies a pinching force along only a small area of the convex surface of the article adjacent its extreme end of said wall to exert a stretching force along the surface of the article.

12. An assembly in accordance with claim 11 wherein said drying forms are made of pressed sheet aluminum.

References Cited by the Examiner UNITED STATES PATENTS 6/1943 Kyle 34-145 2/1960 Friedman 162-224 

1. A METHOD OF ASSEMBLING FREE DRIED PULP MOLDED ARTICLES AND DRYING FORMS FOR THE FORM DRYING OF THE PULP MOLDED ARTICLES TO PREVENT WARPING COMPRISING PLACING THE CONCAVE SURFACE OF A PULP MOLDED ARTICLE ADJACENT THE COMPLEMENTARY CONVEX SURFACE OF A THIN PERFORATED DRYING FORM, PLACING A SECOND DRYING FORM OVER THE ARTICLE TO CONTACT AND APPLY A PINCHING FORCE TO THE ARTICLE ALONG ONLY A SMALL AREA OF THE CONVEX SURFACE OF THE ARTICLE ADJACENT ITS EXTREME END TO EXERT A STRETCHING FORCE ALONG THE SURFACE OF THE ARTICLE, AND ASSEMBLING IN A STACK A PLURALITY OF FORMS AND ARTICLES IN ALTERNATING RELATIONSHIP. 